Pelleting was introduced into the United States feed industry in the mid-1920's to improve feed utilization, increase the density of the feed and improve handling characteristics. Prior to about 1930 several different types of pelleting machines were utilized. Toward the end of the 1920's the so-called "flat die" pelleting machine and "ring die" pellet mill were developed in some of their prior forms. While the flat die machine is still in use in certain applications, the ring die pellet mill quickly became the preferred design and was quickly adopted by the animal feed industry and remains the form of pelleting machine of choice today. In addition to the ring die pellet mill itself, auxiliary equipment was developed including conditioners, cooler/dryers, and related process equipment.
The early pelleting process involved mixing the feed ingredients and pelleting them with no further treatment. The rationale for this approach was to prevent alterations to vitamins and proteins due to the addition of heat to the feed mix. In the late 1930's some processors began subjecting pellet-forming mixtures of animal feed to water and steam by passing the mixtures through a conditioner prior to introduction into the pellet extruders. The addition of steam improved production rates, reduced die wear, and improved pellet quality. Steam conditioning was quickly adopted by the industry and has remained an integral part of the pelleting process to the present time.
In the conditioning step, live steam is injected into the feed mash as it is conveyed through the conditioner which generally consists of a cylindrical tube with a rotating shaft upon which numerous paddles or picks are mounted. The condensing steam increases the temperature and moisture content of the mash. Since the steam is injected directly into the feed mash, the boiler treatment chemicals must be FDA approved.
The focus on research into the pelleting process since the 1960's has been on improving the conditioning operation, with emphasis on increasing the retention time and increasing the temperature to which the mash is conditioned. One of the more recent developments was a pressure pelleting system in which the conditioner and pelleting die cavities were pressurized. This allowed use of higher temperatures and longer conditioning times to improve pellet durability and increase the production rate. However, the use of increased temperatures and conditioning times militates against the inclusion of heat sensitive or labile ingredients which are desirable in complete animal feeds.